This application is related to Japanese application No. 2000-270252 filed on Sep. 6, 2000, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to an organic LED (light emitting diode) display device of active matrix drive type and to a fabrication method therefor. More specifically, the invention relates to an organic LED display device of active matrix drive type employing thin film transistors and the like and to a fabrication method therefor.
2. Description of the Related Art
A known technique for displaying a motion picture on an organic LED display device is based on simple matrix driving (for example, Japanese Unexamined Patent Publication No. 2-37385 (1990), and xe2x80x9cImportant Issues and Commercialization Strategy for Organic LED Devicexe2x80x9d p.55).
In the aforesaid driving method, scanning lines are sequentially driven. For driving a greater number of scanning lines (e.g., several hundreds scanning lines), an instantaneous luminance of several hundred thousands to several millions cd/m2 is required, and the driving method encounters the following problems.
(1) A higher driving voltage is required, so that interconnections suffer from a greater voltage drop.
(2) The driving is required in a lower luminous efficiency area in a higher luminance region, so that power consumption is increased.
To solve these problems, organic LED display devices have been developed which employ thin film transistors for active matrix driving (for example, Japanese Unexamined Patent Publications No. Hei 7(1995)-122360, No. Hei 7 (1995)-122361, No. Hei 7 (1995)-153576, No. Hei 8 (1996)-241047 and No. Hei 8(1996)-227276, xe2x80x9cImportant Issues and Commercialization Strategy for Organic LED Devicexe2x80x9d p.62, IEDM98 p.875).
The organic LED display devices employing the active matrix driving method can be driven at a lower voltage. Since the driving in the higher luminous efficiency area is possible, the power consumption can significantly be reduced. Thus, the active matrix organic LED display devices are superior to the simple matrix organic LED display devices.
However, the organic LED display devices employ two or more thin film transistors for driving each pixel and, therefore, have a significantly reduced aperture ratio as compared with liquid crystal display devices (Japanese Unexamined Patent Publication No. Hei 7(1995)-111341, SID98 p.11).
To increase the pixel aperture ratio, there has been proposed a display structure such that an organic LED device including a pixel electrode, an organic LED layer and a counter electrode overlies thin film transistors with the intervention of an insulating film, and light emitted therefrom is outputted from an opposite side of a substrate (for example, Japanese Unexamined Patent Publication No. Hei 10(1998)-189252).
For patterning of an organic layer (typically, a light emitting layer) for formation of an organic LED layer, there have been proposed a masked evaporation method (Japanese Unexamined Patent Publication No. Hei 8(1996)-227276) and an ink jet method (Japanese Unexamined Patent Publication No. Hei 10(1998)-12377).
With the masked evaporation method, however, it is very difficulty to produce LED devices on a greater size substrate. Further, the ink jet method requires much time for producing LED devices on a greater size substrate.
To cope with these problems, there have been proposed a transfer method (Japanese Unexamined Patent Publications No. Hei 10(1998)-208881 and No. Hei 11(1999)-260549) and a printing method (Japanese Unexamined Patent Publication No. Hei 11(1999)-273859) as a patterning method which can significantly reduce the time required for the production of the LED devices on the greater size substrate.
Where the organic LED layer of the organic LED display device has an extremely small thickness on the order of several hundreds nanometers or smaller, however, undulations on an underlying layer attributed to the presence of the thin film transistors or the presence of contact holes, which will pose no problem in the case of a liquid crystal layer, may cause an inter-electrode short, degradation of the organic LED layer due to field concentration, and deterioration in display quality.
Particularly, where the printing method is employed for the formation of the organic LED layer, even slight undulations on a substrate to be formed with the organic LED layer may produce gaps between a transfer plate and the substrate, so that the transfer plate cannot be brought into intimate contact with the substrate. Therefore, the organic LED layer suffers from a printing failure and variations in thickness. This may also cause an inter-electrode short, degradation of the organic LED layer due to field concentration, and deterioration in display quality due to variations in luminance.
Where the transfer method is employed for the formation of the organic LED layer, the slight undulations prevent an organic LED layer formed on a transfer base film from being brought into intimate contact with the substrate, thereby causing the aforesaid problems.
In view of the foregoing, the present invention is directed to an organic LED display device of active matrix drive type which is free from an inter-electrode short, degradation of an organic LED layer due to field concentration, and deterioration in display quality due to variations in luminance, which may otherwise occur due to undulations on a substrate, a pixel electrode or the like. The invention is further directed to a method for fabricating such a display device.
In accordance with one aspect of the present invention, there is provided an organic LED display device of active matrix drive type comprising: a substrate; a thin film transistor provided on the substrate; a planarization film provided over the thin film transistor; and an organic LED device provided on the planarization film, and comprising a pixel electrode electrically connected to a drain electrode of the thin film transistor via an interconnection extending through the planarization film, an organic LED layer comprising at least one organic light emitting layer, and a counter electrode; the thin film transistor being operative to control one of a current and a voltage to be applied to the LED device; wherein the interconnection has an end face flush with a surface of the planarization film on a pixel electrode side.
In accordance with another aspect of the present invention, there is provided a method for fabricating an organic LED display device of active matrix drive type which comprises an organic LED device and a thin film transistor connected to the organic LED device for controlling a current or a voltage to be applied to the LED device, the method comprising the steps of: forming the thin film transistor on a substrate; forming a planarization film over the thin film transistor; forming a contact hole in the planarization film; forming an interconnection in the contact hole so as to has an end face flush with a surface of the planarization film; forming a pixel electrode on the planarization film formed with the interconnection; forming an organic LED layer including at least one light emitting layer on the pixel electrode; and forming a counter electrode on the organic LED layer.